Refrigeration or cooling systems generally use a single refrigerant in a vapor compression cycle. In such a case, the phase change of the refrigerant in the evaporator and in the condenser will be at constant temperature for all practical purposes.
In the usual case, a mismatch leading to poor performance from the efficiency standpoint occurs. For in general, the heat source stream (the stream being cooled) in the evaporator and the heat sink stream (the stream cooling the refrigerant) in the condenser exchange heat sensibly, that is, without regard to the latent heat of fusion and/or vaporization of the material forming such heat streams. As a consequence, as the heat source stream passes through the evaporator, its temperature continuously decreases while as the heat sink stream passes through the condenser, its temperature continually increases, both toward the temperature value of the system refrigerant at that particular location in the system.
As is well known, the rate of heat transfer in a given system is proportional to the temperature differential with the result that as heat source stream or heat sink stream temperatures approach refrigerant temperature, the rate of heat transfer slows.
In order to avoid insufficient rates of heat transfer, such systems have conventionally utilized relatively large blowers or fans to rapidly move the heat sink stream through the condenser to maintain desirably high temperature differentials. Of course, work must be expended to generate the relatively high flow rates of such fluid streams and such has a negative effect on system efficiency.
In order to overcome these difficulties, in my commonly assigned U.S. Pat. No. 4,598,556 issued July 8, 1986, there is disclosed a system that minimizes the work required to direct a heat sink fluid across the condenser while maintaining a sufficient temperature differential to obtain good heat exchange to thereby increase system efficiency. When used to its maximum efficiency, the system of my patent will employ a pressure ratio of six or more (pressure ratio being defined as the ratio of compressor discharge pressure, absolute, to compressor suction pressure, absolute). This in turn requires, as a practical matter, the use of a multiple stage compressor, which is to say, incoming vapor is first compressed in one compressor stage and then passed to a second compressor stage where it is further compressed, etc.
Multiple stage compressors are, of course, more expensive and complex than single stage compressors. In addition, they are generally of considerably greater weight; and this factor mitigates against their employment in systems that are intended to be utilized, for example, in airborne vehicles.
In addition to my previously identified patent, attention is directed to U.S. Pat. No. 3,768,273 issued Oct. 30, 1973 to Missimer. Missimer discloses a vapor compression cooling system especially designed for obtaining extremely low temperatures. The same has some features in common with my previously identified patent, though it lacks the specific arrangement of heat exchangers that provides for improved efficiency in my prior patent. Missimer is also required to utilize a multiple stage compressor because it utilizes pressure ratios of ten or more.
Thus, the present invention is intended to provide a high efficiency vapor compression cooling system operating on a pressure ratio of four or less enabling the use of a single stage compressor.